3 Answers
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M42 prime lenses are single focal length lenses that fit the M42 mount, also known as screwmount. There are many M42 bodies other than the Zeiss Contax, but no digital M42 bodies.

Beware that Contax mount lenses are typically not M42 mount, but it's replacement, the C/Y mount or even N-mount.

The reason why primes are recommended is because when these were made, zoom lenses were of particularly poor quality.

Any EF-m42 adaptor should work just fine. I am not positive, but some may result in the aperture and distance markings to be at 6 o'clock instead of 12.

You will have manual focus and manual aperture control (you adjust the aperture ring and the lens stops down). Some bodies will allow more or less functions like metering, Av mode, focus-trap, IS.

Some lenses will have an Auto/Manual switch or have preset rings, which can be very helpful. In Auto (preset ring set one way), it will be wide open, allowing you to compose, focus, etc. Then switch to Manual and the lens will stop down to whatever the Aperture ring is set to. Then you shoot. Very helpful if you are using extension tubes and doing macros.

While there are tons of great M42 primes, there are also a lot of bad ones. Look up reviews in the vintage section here.

M42 mount is often referred to as the Pentax screw mount, because it was used on Pentax SLRs before they switched to their bayonet mount. Many people now assume that it was Pentax specific, but it wasn't. Pentax just popularised it.
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thomasrutterFeb 25 '11 at 4:49

I am a total noob when it comes to legacy lenses, so it was news to me that there are lenses with auto/manual switches. I have an ancient M42 lens which predates the automatic aperture era, so it has no aperture pin on the mount, and no auto/manual switch on the lens. I also have a 1987 Canon FD lens which has an automatic aperture, but where the auto/manual switching is done via the mount, and so in my case by a switch on the adapter, rather than on the lens. But a quick google confirms that there is indeed a middle option of a lens with an automatic aperture, and the switch on the lens. Huh!
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Tom AndersonOct 12 '13 at 10:06

Just to add to jrista's point, the M42 mount lenses can often be found quite cheaply and many of them are extremely good, especially ones made by Zeiss. Pentax popularized this mount enourmously in its day, so there is also usually a lot of Pentax lenses available in this mount, also quite good. Which reminds me, you may sometimes see them referred to as "Pentax screw/thread mount" in adverts as a result of the popularization, however it's M42.

Anyways, for reviews and other good info on manual lenses, the Manual Focus Lenses site has a bunch of info and reviews, though not all lenses are M42.

Finally, bear in mind, these lenses will be fully manual and so you need to use them in manual mode on your T2i, possibly including metering (I'm not sure how to do stop-down metering on the Canon), and definitely including focusing.

so What range do I get with these lenses? just 42mm prime? I already have a canon 50/f1.8
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CodeToGloryAug 8 '10 at 20:07

42 is the size of the mount, which has nothing to do with focal length. For example, you can get Pentacon 135mm f/2.8 with M42 mount.
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cheAug 8 '10 at 20:14

@CodeToGlory: The 42mm refers to the diameter of the mount, not the focal length. You can get a wide variety of focal lengths for this mount, and they seem to be priced pretty well. They probably won't be as versatile as a lens designed specifically for your camera, but they would be cheaper.
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jrista♦Aug 9 '10 at 18:45

The M42 is a standard lens mount. It has a 42mm diameter, hence the name. It was apparently originated by Zeiss, however it appears to have extremely broad compatibility with both old and obsolete as well as modern and current camera bodies. This includes Canon EOS, such as a T2i.

It should be noted that to use this mount on camera bodies other than the Zeiss Contax, you will likely require a mounting adapter. Additionally, these lenses do not include any electronic capabilities (or the adapter does not support electronic adaptation), so you will not have any features like auto-focus or electronic aperture control. Everything would be manual if you used one of these lenses on a modern electronic camera.

This might not be obvious: you'll get no autofocus and no in-camera aperture control on M42 lenses, since it has no coupling for such things.
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cheAug 8 '10 at 19:39

There were two aperture control mechanisms used on various M42 lenses: automatic and half-manual. The automatic mode had a little pin on the lens and a corresponding pusher on the body. The aperture ring in fact just sets the aperture closing limit on the lens; the body is pushing full throttle during exposure. The half-manual mode was found on a few soviet lenses. The lens had two aperture rings. One sets the limit in steps, another is a free floating ring that actually closes the aperture till the stop. So you close the aperture before shooting and open back when looking for a shot.
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egorFiNEAug 8 '10 at 23:46

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There are a plethora of camera bodies with the M42 mount other than the Zeiss Contax. @egorFiNE: Most, if not all adapters do not push the little pin on the edge to get automatic aperture working. Instead, the Aperture/Manual switch works as the preset aperture lenses (which you call half-manual). See my answer.
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EruditassAug 9 '10 at 18:53

I have pre-automatic Pentacon 135 mm f/2.8. There is no aperture pin, and only one aperture ring, which is clickless. However, the aperture ring is a bit funky, it than you can push it forward to decouple it from the aperture scale, rotate it relative to that, then drop it back to recouple it. There is a limit stop which applies to the ring (the bit you can decouple) rather than the scale, so you can set a minimum aperture. You can then flip between wide open and the setting by cranking the ring. Bit hard to explain, but i can make a video if anyone's interested!
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Tom AndersonOct 12 '13 at 10:18